Along with the miniaturization of LSIs, there are hopes for the commercialization of electron beam projection lithography (EPL). As the EPL being commercialized, PREVAIL (projection exposure with variable axis immersion lenses) technology jointly announced by IBM and Nikon can be mentioned. PREVAIL technology is described for example in H. C. Pfeiffer et al., Journal of Vacuum Science and Technology, B17, p. 2840 (1999).
Further, LEEPL (low energy electron-beam proximity projection lithography) technology jointly announced by LEEPL Corporation, Tokyo Seimitsu Co., Ltd., and Sony can be described as another EPL. LEEPL technology is described for example in T. Utsumi, Journal of Vacuum Science and Technology, B17, p. 2897 (1999).
As the PREVAIL mask and the LEEPL mask, stencil masks have been proposed. Stencil masks have a thin film dispersing, reflecting, or absorbing exposure beams such as electron beams and ion beams and holes formed with predetermined patterns for passing through the thin film. Since the exposure beam passes through the portions of holes passing through the thin film and is dispersed, reflected, or absorbed at the thin film except at the holes, a predetermined pattern is exposed. In stencil masks, the thin film formed with such holes is called a “membrane”.
PREVAIL is generally a 4× reduction projection system. An electron beam of about 100 keV is used. When using a PREVAIL stencil mask for exposure, an electron beam is passed through only portions formed with holes without dispersal and is focused on a resist film to transfer a pattern. As the PREVAIL mask, a stencil mask having a 2 μm thick membrane composed of silicon has been proposed.
On the other hand, LEEPL is an equal magnification projection system. An electron beam of for example 2 keV is used. When using an LEEPL stencil mask for exposure, an electron beam is passed through only portions formed with holes to transfer a pattern at equal magnitude. As the LEEPL masks, stencil masks having 500 nm thick membranes composed of silicon or diamond have been proposed.
However, according to the above conventional stencil masks, when reducing the membrane thickness for the purpose of improving the processing of the mask to form a finer pattern, the mask strength falls. In particular, LEEPL stencil masks are thin in membrane thickness and fine patterns of equal magnitude as the transferred patterns are formed, so the drop in the mask strength easily becomes a problem.
In the case of using silicon as the membrane material, compared with the case of using diamond, the drop in the mask strength is more conspicuous. If the mask strength is insufficient, for example, when washing the masks or loading the masks at an exposure apparatus, the patterns easily break.
Further, if the area of the membrane is large, when making the membrane thickness thinner, the membrane easily flexes. If the membrane flexes, the positional precision of the pattern will drop or the pattern will become distorted. To form a fine pattern on a semiconductor device, it is necessary to prevent flexing and distortion of the membrane.